Diaphragm electrolytic cell

ABSTRACT

A diaphragm electrolytic cell is composed of two or more overlaid modules; at least the upper modules having U-shaped anodes with diaphragm-coated cathodes housed within, allowing for a reduced electrode pitch.

This application is a 371 of PCT/EP03/01977 filed Feb. 26, 2003.

BACKGROUND OF THE INVENTION

The world-wide production of chlorine, about 45 million tons per year,is carried out in electrolytic cells of different types; among these,the diaphragm electrolytic cell, by means of which about 22 million tonsof chlorine per year are produced, has a great relevance.

A diaphragm electrolytic cell is generally composed of four main parts,as known to the experts in the art: a copper anodic base, lined with aprotective titanium sheet, an anodic package, consisting in amultiplicity of anodes disposed in parallel rows and secured to saidbase, a carbon steel cathodic body, comprising a plurality of cathodesupon which a porous diaphragm is deposited, secured to a currentdistributor and disposed in parallel rows so that they can beintercalated to the above anodes according to a so-called “finger-type”geometry, and a cover, usually of chlorine-resistant plastic materialprovided with the nozzles for feeding the brine and discharging theproduct chlorine.

In consideration of the high number of installed cells (about 25000world-wide), of the high amount of energy involved in their operation(about 60 millions of MWh/year) and of the continuous increase in thecost of electricity, the cell diaphragm technology has been, in thecourse of the years, remarkably improved. Among the many technologicalinnovations which offered the major contributions for decreasing theenergy consumption, the following must be noticed:

-   -   the replacement of the traditional graphite anodes with        box-shaped perforated metallic anodes (the so-called “box” type        anodes) made of titanium, coated with electrocatalytic material        based on noble metals and/or oxides thereof.    -   the replacement of fixed sized “box” anodes with the so-called        “expandable anodes”, as disclosed in U.S. Pat. No. 3,674,676,        allowing for the reduction of the interelectrodic gap.    -   the suppression of the above interelectrodic gap through the        introduction, within the expandable anodes, of means for        exerting a pressure between the anodes and the diaphragm, as        disclosed in U.S. Pat. No. 5,534,122    -   the evolution of the expandable anode through the introduction        of the double expander, as disclosed in U.S. Pat. No. 5,993,620,        whereto a lower ohmic drop is associated.

It may be observed that the cited innovations are all directed toimprove the performances in terms of energetic consumption, by means ofeither an increase of the electrocatalytic activity, or an optimisationof the electrode structure, or again through the reduction of theinterpolar gap and the increase in the mass transfer (lower bubbleeffect and higher electrolyte circulation) obtained through smallmodifications which do not imply a substantial redesign of the cellstructure and thus of easy implementation and reduced costs.

Other solutions proposed in the past provide a modification of the cell,and in particular of the cathodic package, directed to increase theelectrodic surface thereby decreasing the current density at a givenapplied total current, and as a consequence the cell voltage and theoverall energetic consumption.

A further issue of present great relevance is given by the need ofincreasing the electric load and thus the production; such need is oftenin contradiction with the lack of a suitable area allowing theinstallation of additional electrolytic cells. In the co-pendingunpublished International Application PCT/EP 02/10848, a solutionallowing the increase of the cell active surface with the same projectedarea is disclosed, by means of the construction of a cell made of aplurality of vertically overlaid modules provided with the conventionalinterdigitated anodes. This solution is in itself promising, althoughentailing quite substantial investment costs.

It is an object of the present invention to provide a new diaphragmelectrolytic cell overcoming the drawbacks of the prior art.

In particular, it is an object of the present invention to provide adiaphragm electrolytic cell comprising a multiplicity of overlaidmodules of anodes and cathodes, the anodes of at least part of themodules allowing for a substantial reduction of the construction cost.

SUMMARY OF THE INVENTION

The invention consists of a diaphragm electrolytic cell made of a lowermodule and of an upper module or a multiplicity of upper modulesvertically overlaid thereto, wherein at least the upper modules areprovided with generally U-shaped anodes, comprising two vertical majorsurfaces fixed to a horizontal current collector, housing thecorresponding cathodes within.

The two vertical major surfaces of the anodes may be part of a singlefolded surface; they are preferably foraminous, to allow the circulationof the electrolyte, and are preferably provided with an electrocatalyticcoating for chlorine evolution.

In order to facilitate the understanding of the invention, referencewill be made to the attached figures, which are not to be intended aslimiting the invention itself, whose domain is solely limited by theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side view of a diaphragm electrolytic cell of the priorart.

FIG. 2 shows an anode of the cell of the invention according to a firstpreferred embodiment.

FIG. 3 shows an anode of the cell of the invention according to a secondpreferred embodiment.

FIGS. 4A and 4B show an arrangement of anodes and cathodes in a moduleof the diaphragm electrolytic cell of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diaphragm electrolytic cell of the prior art, accordingto the teaching of the co-pending non published InternationalApplication PCT/EP 02/10848. The illustrated cell consists of twovertically overlaid modules, an upper module (100) and a lower module(200), according to the most common embodiment; it is intended that theupper module (100) may be replaced by a plurality of vertically stackedupper modules, as disclosed in the cited co-pending application. Thelower module (200) comprises a copper anode base (1), lined with atitanium protective sheet (not evidenced) whereto a plurality of anodes(3) is secured in parallel rows, by means of current collecting stems(4) intercalated to the cathodes (5). The surface of the anodes ispreferably made of a grid of perforated sheet or rhomboidal-shapedexpanded sheet coated with an electrocatalytic material. The cathodicpackage consists of a box (6) with open top and bottom, known ascathodic body, with a current distributor (30), provided with aplurality of cathodes (5) fixed inside, secured in correspondence of theexternal surface thereof. The cathodes (5), known as fingers, are shapedas tubular boxes with a flat elongated cross-section and are arranged inparallel rows intercalated to the rows of anodes (3); the two ends ofthe cathodes (5) are connected with a manifold (7) running along thefour sides of the box (6). The cathode is made for example of an ironperforated sheet or mesh, with the diaphragm deposited on the externalsurface thereto, facing the anode. The diaphragm has the purpose ofseparating the anodic compartment from the cathodic one avoiding themixing of the two gases and of the solutions; originally it was made ofpolymer modified asbestos, but the technological evolution has led tothe adoption of composite asbestos-free diaphragms. The diaphragm mayalso consist of an ion-exchange membrane or other semipermeablematerial. The upper module (100) also comprises anodic and cathodicpackages, substantially with the same construction materials as in thelower module (200) but in most of the cases of lower height. The upperanodic package is comprised of a frame (15), acting as the upper anodicbase and ensuring for the mechanical support and the currentdistribution for the relevant anodes (16). The frame (15) is made of atitanium sheet provided with holes or slots, suitably dimensioned forputting the two anodic compartments in direct fluid communication. Theanodes (16) of the upper module are vertically fixed to the frame, intransversal rows, generally with the same pitch as in the lower module.The anodes (16) of the upper module, fixed to the frame (15) by means ofdowel screws (18), often have a lower height. The upper cathodic body ismade of a box (19), having the same design and construction materials asthat of the lower module and a height depending on that of the upperanodic package; the upper cathodic body is welded along the internalwalls of the box (19) to a plurality of cathodes (20) arranged inparallel rows. Each finger, shaped as an elongated tubular box, is incommunication with a manifold (21) positioned along the sides of the box(19). The main features of the cathodes and diaphragms of the uppermodule are equivalent to those of the lower module. The frame (15) andthe anode base (1) are reciprocally connected by means of externalconductors (not shown in the figure); the box (6) and the box (19) arealso connected in a similar way. The cell cover (8), which is made a ofplastic chlorine-resistant material, is provided with a chlorine gasoutlet (9) and a brine inlet (10). The cell is connected to a directcurrent supply by means of bus bars. As known to the experts in the art,the cell operates as follows: the feed brine enters the cell through theinlet nozzle (10) placed on the cell cover and is distributed throughpipe (23) to the base (1) of the lower anodic compartment, subsequentlyrising to the top surface thereof and overflowing through the slots ofthe frame (15) into the anodic space of the box (19). The chlorinedisengaged in the lower anodic compartment follows the same path andleaves through the outlet nozzle (9) on the cover (8). Thechloride-depleted electrolyte, driven by the pressure corresponding tothe hydraulic head between the anolyte and catholyte, permeates throughthe diaphragm entering the upper (20) and lower (5) cathodic fingers.Hydrogen leaves the upper (21) and lower (7) cathodic compartmentsrespectively through nozzles (25) and (11), connected in parallel to thehydrogen manifold (26). The alkali produced in the upper cathodiccompartment (21) leaves through nozzle (27), and enters the lowercathodic chamber (7) through pipe (28) and nozzle (29), where it ismixed with the alkali produced therein, then leaving the cell throughthe hydraulic head (12). The level of the cathodic liquor is normallyadjusted so that a sufficient gas chamber is always maintained in thelower cathodic compartment (7); consequently, the upper compartment (21)works exclusively as a gas chamber and electrolysis takes place only bydirect contact between the solution percolating onto the diaphragm andthe cathode. To establish such condition in a reliable fashion, the pipe(28) must obviously have a sufficient large diameter in order to remainsubstantially full of hydrogen, so that the two cathodic compartments(7) and (21) are subjected to an identical pressure.

FIG. 2 shows a particular embodiment of the anode of the invention,which is conceived in a completely different manner with respect to theprior art anodes, with or without expander. As it can beobserved, theanode structure is given by an electrodic surface (13), folded and openon one side to allow the insertion of a cathode, preferably consistingof a foraminous sheet or a mesh or, as an alternative, of ajuxtaposition of foraminous elements such as sheets or meshes. The anodehas a single curvature (14), his profile thereby assuming a U-shapedgeometry; other kinds of curvature are however possible withoutdeparting from the scope of the invention. At the base of the anode, incorrespondence of the curvature (14), a current collector (150) providedwith a preferably threaded stem (160) is welded or otherwise secured.The current collector (150) is horizontal instead of vertical as itwould be the case of the prior art, as this allows the internal volumeof the anode to be hollow and completely available for the insertion ofthe corresponding cathode. In principle, both the anodes of the uppermodule (100) and of the lower module (200) could be realized accordingto the embodiment of FIG. 2. However, the cell construction illustratedin FIG. 1 derives, in most of the case, from a retrofitting of an olderdiaphragm cell wherein the upper module is overlaid to the lower one ina second time, as disclosed in the co-pending International ApplicationPCT/EP 02/10848. The anodes of the lower module (200) have therefore, inmost of the cases, a geometry according to the prior art. Also when acomplete replacement of the electrodes of the lower module is carriedout, the advantage of employing the anode of FIG. 2 is partiallycounteracted by the fact that the anodes (3) of the lower module (200)are usually quite high (for instance 800 mm); the lack of an internalcurrent collector way entail, in this case, substantial ohmic penaltiesthus lowering the faradaic yield. The anodes (16) of the upper module(100) have conversely a much reduced typical height (for instance 160mm, as specified in the cited International Application PCT/EP02/10848), and conducting The electric current along their whole heightwithout resorting to internal current collectors is therefore anegligible issue. For this reason, in a preferred embodiment, the cellof the invention makes use of the anodes of FIG. 2 only for the uppermodule (100).

In another embodiment, the cell of the invention makes use of suchanodes also for the lower module (100), counteracting the increase inthe ohmic drop along the electrode height with additional verticalcurrent collectors (not shown), secured to the external surfaces of theanodes. The optional additional titanium-lined copper currentcollectors, secured externally and not internally, are much easier toremove and restore, contributing in a sensible manner to reduce thecosts of reactivation.

Fixing the current collectors to the anodes externally instead ofinternally also offers an additional benefit: when the catalytic coatingis periodically deactivated, the anode must be in fact subjected to areactivation, preceded by an etching treatment in hot concentratedhydrochloric or sulphuric acid. After applying the catalytic ink, theanode must be treated in oven at about 500° C. During these treatments,the bimetallic contact between the copper core of the state-of-the-artcurrent collector and the relevant titanium lining would be seriouslydamaged by distortion phenomena, the previous detachment of the currentcollector and his subsequent restoring after the treatment beingtherefore required. With the illustrated anode however, the horizontalcurrent collector can be entirely made of titanium, with littleprejudice in terms of ohmic drops, therefore no problems arise duringthe heat treatment of reactivation.

FIG. 3 shows a second particular embodiment of the anode of theinvention, whose conception is not too far from the anode of FIG. 2.Once more, its structure is open allowing the cathode to be housedwithin; in this case, however, the electrodic surface (13) is formed bytwo distinct elements, disposed in the vertical position and secured tothe current collector (150) in correspondence of one edge (17). Thenature of the electrodic surface (13) is fundamentally equivalent to theone described for the previous embodiment; the use of foraminouselements such as sheets or meshes, or juxtapositions thereof, ispreferred.

FIG. 4 is a sketch of a side view of a possible configuration of anupper module (100), according tote best mode of carrying out theinvention; the same configuration could be used for the lower module(200), without departing from the scope of the invention. The particularshape of the anode (16), with an open upper part and the interior freeof obstacles, maybe exploited for housing the cathode (20) within, sothat the reduction of the electrode pitch is virtually limited by thesole thickness of the cathode (20). The adjacent anodes, m fact, can bevery close to each other as even in mutual contact, as they aremaintained at the same electric potential. The figure shows alsoconstraint elements (31), applied to adjacent pairs of anodes, that areused to open wide the latter under elastic regime so as to facilitatethe insertion of the cathodes during the assembly (FIG. 4A); FIG. 4Bshows how, upon completing the assembly and removing the constraintelements, the anodic surface moves back to the natural position, withthe two vertical sides facing the diaphragm-coated major surfaces of thecorresponding cathode (20). In FIG. 4, the anode (16) has an open upperpart, but it is clearly possible to assemble the anodes upside down,with an open lower part. It is also possible to provide an assemblyprocedure that doesn't make use of constraint elements, or that utilisesthe same in a different fashion, without departing from the scope of theinvention. The constructive solution illustrated in FIG. 4 easily allowsan increase of active surface of 30-50% for the relevant module and fora given projected surface.

In the description and claims of the present application, the word“comprise” and its variation such as “comprising” and “comprises” arenot intended to exclude the presence of other elements or additionalcomponents.

1. A diaphragm electrolytic cell for the electrolytic production ofchlorine and alkali comprising a lower module equipped with a loweranodic package and a lower cathodic package and at least one uppermodule overlaid thereon equipped with an upper anodic package and anupper cathodic package, said modules being hydraulically connected inseries, said hydraulic connection in series comprising an externalmanifold for the product alkali and a direct fluid communication betweensaid upper anodic package and said lower anodic package by means ofholes or slots provided in a conductive frame acting as a base for saidupper anodic package, wherein at least one of said modules is equippedwith U-shaped anodes comprising two vertical major surfaces fixed to afirst horizontal current collector, additional titanium-lined coppercurrent collectors being externally secured to at least one of saidvertical major surfaces, diaphragm-coated cathodes being housed in thehollow space inside said vertical major surfaces.
 2. The cell of claim 1wherein said vertical major surfaces of said anodes are parts of asingle folded surface delimited by a curvature, said horizontal currentcollector being fixed to said vertical major surfaces in correspondenceof said curvature.
 3. The cell of claim 1 wherein at least one of saidvertical major surfaces of said anodes is foraminous.
 4. The cell ofclaim 3 wherein said at least one foraminous vertical major surface ofsaid anodes consists of a foraminous sheet or of a mesh or of ajuxtaposition of foraminous sheets or meshes.
 5. The cell of claim 1wherein said U-shaped anodes are entirely made of titanium or titaniumalloys.
 6. The cell of claim 1 wherein said at least one upper module isequipped with U-shaped anodes.
 7. The cell of claim 4 wherein saidU-shaped anodes are entirely made of titanium or titanium alloys.
 8. Thecell of claim 1 wherein said at least one upper module is equipped withU-shaped anodes.
 9. The cell of claim 3 wherein said at least one uppermodule is equipped with U-shaped anodes.
 10. The cell of claim 5 whereinsaid at least one upper module is equipped with U-shaped anodes.
 11. Thecell of claim 1 wherein said horizontal current collector is made oftitanium.
 12. The cell of claim 3 wherein said horizontal currentcollector is made of titanium.
 13. The cell of claim 4 wherein saidhorizontal current collector is made of titanium.
 14. The cell of claim11 wherein all of said lower and upper modules are provided with saidU-shaped anodes.
 15. The cell of claim 12 wherein all of said lower andupper modules are provided with said U-shaped anodes.
 16. The cell ofclaim 13 wherein all of said lower and upper modules are provided withsaid U-shaped anodes.
 17. The cell of claim 1 wherein at least saidvertical major surfaces are provided with a catalytic coating forchlorine evolution.
 18. The cell of claim 3 wherein at least verticalmajor surfaces are provided with a catalytic coating for chlorineevolution.
 19. The cell of claim 4 wherein at least said vertical majorsurfaces are provided with a catalytic coating for chlorine evolution.20. The cell of claim 5 wherein at least said vertical major surfacesare provided with a catalytic coating for chlorine evolution.
 21. Amethod for the assembly of the cell of claim 1, comprising fixing saidU-shaped anodes to the corresponding anodic base, maintaining saidanodes open wide under an elastic regimen by means of constraintelements, housing said diaphragm-coated cathodes within said hollowspace inside said vertical major surfaces of said anodes and removingsaid constraint elements.
 22. A process for the production of chlorineand caustics comprising applying a direct electric current to a cell ofclaim 1 fed with an alkali chloride solution.
 23. The process of claim22 wherein said caustics comprise sodium hydroxide and said alkalichloride solution comprises sodium chloride brine.